The invention relates to a motor vehicle fuel supply system with a deventilating device for a fuel tank fillable via a filler pipe, with an outwardly gas-tight deventilating path between the tank and a filter chamber for a regenerable filter medium retaining fuel vapors, as well as with a ventilating orifice arranged on the filter chamber and with an outwardly gas-tight connecting path which connects the filter chamber to an underpressure source, for example the vehicle engine, and which can be shut off by a valve (regenerating valve).
A deventilating device of this type is known from German Patent Document DE 2,416,067 A1. In this device there is a valve assembly arranged on the ventilating orifice which closes the ventilating orifice only when an underpressure which is lower than a predetermined threshold value prevails in the filter chamber in relation to atmospheric pressure.
Moreover, deventilating devices of this type are already used as standard.
The deventilation of the tank can take place only via the filter chamber, in which retained by the filter medium present there, for fuel vapors are example activated charcoal. The filter medium can be regenerated regularly by connecting the filter chamber to the intake side of the vehicle engine, so that the engine receives fresh air via the filter chamber, with the result that the fuel vapors previously absorbed by the filter medium are entrained to the engine and are burnt there.
The requirement has arisen in the meantime to have the capability of checking the gas-tightness of the fuel system and the associated deventilating device by means specific to the vehicle.
An object of the invention is to provide an arrangement which is optimum for the purpose of satisfying this last mentioned requirement.
In a deventilating device of the type specified above this object is achieved according to preferred embodiments of the invention by arranging a valve assembly at the ventilating orifice which closes the ventilating orifice only when an overpressure falling below a threshold value prevails in the filter chamber in relation to atmospheric pressure, and characterized in that this threshold value has approximately the value of that maximum pressure which can be generated by fuel received in the filler pipe. It is also appropriate for the threshold value to also be somewhat slightly above this value according to certain preferred embodiments.
The invention is based on the general idea, on the one hand, of allowing overpressure in the tank system, so that the tightness of the system can be checked quickly. On the other hand, the possible overpressure remains sufficiently limited for it to be possible to rule out any dangerous operating states. Since the possible overpressure is near the maximum pressure which can be generated by fuel in the filler pipe, this ensures at the same time that the automatic cut-off mechanism preventing an overfilling of the tank on fuel-dispensing pumps at the filling stations can work in a fault-free manner.